Having thought about it for a bit, I vaguely recall that the studs and head bolts are not parallel? on these engines, so the head has to be fitted over the studs first and then the head bolts inserted? So, maybe bolts could be used after the head is fitted? I also seem to recall that possibly earlier 128 engines had all 10mm bolts used on the head - but my old memory might be failing me... I do remember there were 2 Fiat special tools needed to tighten/ retighten the head fasteners in situ.
Also, given the hardness of the head bolts, it might not be feasible to cut threads on one to convert it into a stud, I seem to recall that the bolt threads might have been 'rolled' into the material at the factory (makes them less likely to fail where the thread starts) just like spoked wheel spoke threads.
I reckon your best (and cheapest) option is to try to find someone who could let you have some used head studs.